1. Field of the Invention
The present invention relates generally to resistors formed within and upon semiconductor integrated circuit substrates. More particularly, the present invention relates to a high resistivity multi-layer polysilicon resistor for use within semiconductor integrated circuit device cells of limited dimension.
2. Description of Related Art
In addition to the use of transistors and diodes as switching elements and current rectifying devices in semiconductor integrated circuits, it is also quite common in the art that those circuits will have resistors incorporated into their designs and fabrications. Commonly, a resistor structure within an integrated circuit will provide an electrical load which assures proper operation of the circuit under the electrical current and voltage parameters to which the circuit was designed.
There are several methods and materials through which resistors may be designed and fabricated into semiconductor integrated circuits. A common traditional method is to fabricate resistors within the silicon semiconductor substrate of an integrated circuit. Resistors of varying resistance can easily be fabricated within such semiconductor substrates by means of doping the semiconductor substrate through an ion implantation process similar to the ion implantation process used in forming other active semiconductor regions within the same semiconductor substrate. The practice of forming resistors through implanting dopant ions into semiconductor substrates is a viable technique unless resistors are required whose resistance exceeds the resistance of the semiconductor substrate within which the resistor is desired to be fabricated.
An alternate method which provides resistors of greater or equal resistance to resistors formed within semiconductor substrates is to form resistors from discrete portions of a high resistivity material which is deposited upon the surface of the lower resistivity semiconductor substrate. A common high resistivity material which is useful in forming these higher resistance resistors is undoped or lightly doped polysilicon. While it is known in the art that highly doped polysilicon is an excellent conductor from which conductive structures within integrated circuit devices may be fabricated, it is also well known that undoped or lightly doped polysilicon is a highly resistive material.
When formed from a high resistivity material such as undoped or lightly doped polysilicon, the resistance of a polysilicon resistor may be determined from straightforward considerations. Specifically, the resistance of the polysilicon resistor is nominally directly related to the length of the polysilicon layer from which the resistor is formed and nominally inversely related to the cross-sectional area of the polysilicon layer from which the resistor is formed. For resistors formed via planar thin film polysilicon processes within integrated circuits, the resistance of the polysilicon resistor is nominally inversely related to the thickness and the width of the polysilicon layer from which the resistor is formed.
Cross-sectional diagrams illustrating the formation of a typical polysilicon resistor within an integrated circuit are shown in FIG. 1a and FIG. 1b. Within FIG. 1a there is shown a semiconductor substrate 10 upon which there is formed a first insulating layer 12. Upon the first insulating layer 12 is formed a blanket polysilicon layer which is comprised of polysilicon layers 14a, 14b and 14c. The polysilicon layer 14b which resides beneath a patterned photoresist layer 16 forms a polysilicon resistor. The polysilicon layers 14a and 14c are typically subsequently heavily doped through the process of ion implantation to form conductive doped polysilicon ends to the polysilicon resistor which is formed from the polysilicon layer 14b.
FIG. 1b shows a continuation of the fabrication of the integrated circuit within which the polysilicon resistor illustrated in FIG. 1a is formed. Shown in FIG. 1b are patterned second insulating layers 17a and 17b formed upon the exposed surfaces of the polysilicon layers 14a, 14b and 14c after the patterned photoresist layer 16 has been removed. Between the patterned second insulating layers 17a and 17b resides an aperture at the bottom of which is the surface of polysilicon layer 14c. Finally, there is formed upon the exposed surfaces of the patterned second insulating layers 17a and 17b and into the aperture between those two insulating layers a conductor layer 18. The conductor layer 18 makes contact with the polysilicon layer 14c through a conductor/polysilicon contact 20.
As semiconductor technology has advanced, conflicting trends have evolved which require resistors of increased resistance while simultaneously providing diminished semiconductor substrate surface area upon which those resistors may be fabricated. In addition, the thickness of polysilicon layers from which are formed high resistance resistors in integrated circuit devices is nearing the limits with which current semiconductor equipment may efficiently and reproducibly deposit those layers.
Thus, the problem to which the present invention is directed is defining a more efficient high resistance polysilicon resistor structure and its method of fabrication within semiconductor integrated circuits. The desired high resistance polysilicon resistor structure will provide a resistor of sufficiently limited surface area to efficiently use semiconductor substrate surface area available to the resistor. The desired method by which the high resistance polysilicon resistor is formed will not unduly challenge established semiconductor manufacturing processes and process limitations.
Methods for providing variable resistances within polysilicon films formed upon silicon substrates are known in the art. For example, Graeger, et al. in U.S. Pat. No. 5,164,338 describes a method of manufacturing doped polysilicon resistive layers upon silicon substrates for use in pressure sensing devices. The Graeger disclosure describes a two-layered fabrication of doped polysilicon layers upon a silicon substrate. The individual layers are formed of doped polysilicon having differing polysilicon grain sizes. When used within a pressure sensing device, the disclosed fabrication provides a sensor exhibiting increased sensitivity to minute pressure changes.
Not disclosed within the art is a static high resistivity polysilicon resistor of substantially higher resistance than is common in the art. Nor is there disclosed in the art a method by which the resistance of a polysilicon resistor of limited thickness and surface area may be increased without increasing the length or decreasing the thickness of the polysilicon resistor.